Once thought to be boring and well understood, attic ventilation recently received a much-needed reevaluation. The need for new techniques, and even a whole new way of thinking about attics, comes from two main sources: energy efficiency concerns and mold growth issues. Fortunately, in an attic, these problems go hand in hand; solving one problem will often address the other.
Why are attics ventilated?
Attic ventilation attempts to accomplish several key goals.
- Reduce condensation (and mold) on the underside of the roof sheathing in the winter months.
- Reduce solar heat gain inside the house during the summer months.
- Provide a path to dry unexpected water intrusions, roof leaks, etc.
The former is important in cold climates, where ice buildup and condensation can occur if liner temperatures reach dew point. The second is critical in hot climates, where extreme temperatures from attic heat can radiate into the conditioned space and increase the load on the air conditioner.
Active attic ventilation is a fairly new invention, needed only after the advent of more stringent construction techniques and lower quality materials. The first occurred when homes were converted from chipped sheet and cedar shake to plywood and composite roofing. The stir-fried sheets with shakes allowed a significant amount of heat and moisture to escape without the need for specific ventilation. This is why in older turn-of-the-century homes, you’ll very rarely encounter mold problems in the attic. Compounding the problem in recent decades was the shift from real wood to manufactured materials like OSB and plywood. These materials have much lower resistance to mold growth.
The worst problems often occur in homes built in the 1960s, ’70s, and ’80s. Homes built in this era often contain manufactured wood materials, composition shingles, and poor airflow management. Also, these homes tend not to have roof air sealing, which allows a lot of warm, humid air to enter the attic space.
The need for attic ventilation varies greatly from home to home. I have looked at several houses with the exact same design, roof structure and ventilation, but one is overgrown and moldy and the other is immaculate. Although frustrating at first, after a little research, aggravating factors can often be identified. Perhaps the most common variable is the number of roof penetrations. Elements such as can lights, access hatches, etc. they provide a very efficient path for warm, humid indoor air to migrate into the attic space and can have a dramatic effect on condensation and mold growth. There are also invisible penetrations; Wherever an electrical cable penetrates the top plate of a wall, heat and moisture can escape into the attic.
How does attic ventilation work?
Most residential homes rely on passive ventilation to encourage airflow. Passive ventilation requires intake (typically soffit vent) and exhaust (ridge vent or roof intakes). Due to the stack effect, as warm air rises, cool air enters through the soffit vents and exits through the ridge area. Occasionally homes rely on gable vents for passive ventilation. Problems can occur if any of the ventilation components are not working properly.
Common problems include:
- Soffit vents blocked by insulation
- Gabled vents competing for airflow
- The missing crest are vents
- Ridge vents with insufficient airflow (caused by poor design)
- Ridge vents with insufficient trimming of the roof sheathing.
- Hip roofs with limited ridge area.
- Low angle roof with insufficient clearance between the top of the insulation and the roof sheathing.
Air sealing: the surprising solution.
Although attic ventilation is important, the latest research has identified an equal, if not superior, solution. Air sealing. The concept is very simple. We can try to remove heat and moisture from the attic after it arrives, or we can prevent it from getting in in the first place. This is accomplished by air sealing top plates, can lights, access hatches, electrical penetrations, etc. In addition to greatly reducing the chance of condensation and mold growth, a home will typically see significant energy efficiency gains. Sealing a home with air is relatively inexpensive, often cheaper than increasing ventilation or adding a powered roof vent.
When is mechanical ventilation necessary?
Sometimes passive ventilation is insufficient or impractical. Low angle ceilings, for example, have a hard time producing enough airflow and are often dependent on mechanical assistance. Gabled roofs, with a wide footprint and minimal ridge area, also suffer from poor passive ventilation and may require an active system. In a commercial setting, mechanical ventilation is often achieved through wind turbine vents. These work especially well in areas with constant wind. However, they are unsightly for residential applications and most homeowners choose to install power roof vents instead. These mount to the top of the roof or to a gable.
Remember, a powered roof vent still requires sufficient air intake if it has any hope of adequately ventilating the attic space. In fact, if adequate intake air is not provided, the fan will pull warm, humid air out of the house instead of the soffit vent. This would only aggravate condensation and mold problems.
When is a Professional Mold Inspection Necessary? If there is visible mold growth on the underside of the roof sheathing, contact a mold specialist. However, many mold inspectors do not have a good understanding of attic ventilation. Before hiring, make sure they understand attic mold dynamics, humidity, the chimney effect, etc. If they are just going to take a few measurements and identify the type of mold, you are wasting your money.